1. Field of the Invention
The present invention relates to an optical film that has an optically negative uniaxiality and that is suitably used in various applications for optical films, a method of producing the optical film, an optical element or an image display device including the optical film.
2. Description of the Related Art
Liquid crystal displays (LCDs) such as STN (Super Twisted Nematic)-LCDs and TFT (Thin Film Transistor)-LCDs have considerable merits e.g., they are thin, light-weighted, driven at a low voltage, and power-saving. For this reason, such LCDs are used widely for high-quality displays that replace cathode-ray tubes, and the displays are used for portable electronic equipment such as portable TVs, digital cameras, and video cameras; as well as laptop personal computers, and computer monitors. In general, many of these liquid crystal displays include birefringent optical films made of polymer materials or liquid crystal materials in order to solve problems such as display coloring caused by birefringence of liquid crystal cells, deterioration in contrast, and deterioration in the viewing-angle characteristics.
Some birefringent optical films that are being produced or under development have uniaxial refractive indices. Such kinds of optical films are classified further into films having in-plane optical axes and films having out-of-plane optical axes (in the film normal directions). The films are classified still further depending on whether the axes are positive or negative. A typical example of the films having the optical axes in the in-plane of the film is an optical film used for color compensation in a STN-LCD. The optical film for color compensation is obtained generally by stretching a polymer film, and it presents an optically positive uniaxiality.
An example of optical films having the out-of-plane optical axes is an optical film of polyimide presenting an optically negative uniaxiality (hereinafter may be described as xe2x80x98having a negative birefringencexe2x80x99), which is proposed by Harris et al. and Ezzell et al (see, for example, U.S. Pat. Nos. 5,344,916, 5,480,964, 5,580,950, and 6,238,753). Such a film can be produced, for example, by coating a polymer solution on a proper substrate so as to plane-orient the polymer. The thus produced film can be used for an optical film for improving a visual angle of a TFT-LCD.
An object of the present invention is to provide an optical film having excellent heat resistance, high uniformity, and a negative refractive index. Furthermore, by compensating birefringence of a liquid crystal display, the present invention provides an optical element having good color tones and viewing-angle characteristics. In addition, the present invention provides image display devices such as liquid crystal displays and organic EL display devices using the optical elements.
For achieving the above-described objects, the present invention provides an optical film containing polyaryletherketone and having a negative birefringence. Such an optical film is excellent in the heat resistance and uniformity, and has a negative birefringence. It is preferable in the film that the negative birefringence is in a range of 0.001 to 0.6.
It is also preferable in the optical film that the polyaryletherketone is a polyaryletherketone having at least one fluorine atom in a repeating structural unit of its main chain.
It is also preferable that the polyaryletherketone has a repeating structural unit represented by a formula (1): 
where F denotes a fluorine atom; A denotes a halogen atom, a lower alkyl group or a lower alkoxy group; x and y denote integers from 0 to 4; m denotes 0 or 1; n represents a polymerization degree; and R1 denotes a group represented by a formula (2): 
where F denotes a fluorine atom; Axe2x80x2 denotes a halogen atom, a lower alkyl group or a lower alkoxy group; z and xxe2x80x2 denote integers from 0 to 4; p denotes 0 or 1; and R2 denotes a divalent aromatic group. The polymerization degree n is, for example, from 40 to 1000, preferably from 50 to 900, and more preferably from 70 to 800.
It is particularly preferable that the polyaryletherketone has a repeating structural unit represented by a formula (3): 
where F denotes a fluorine atom; x denotes an integer from 0 to 4; m denotes 0 or 1; n represents a polymerization degree; and R1 denotes a group represented by the formula (2): 
where F denotes a fluorine atom; Axe2x80x2 denotes a halogen atom, a lower alkyl group or a lower alkoxy group; z and xxe2x80x2 denote integers from 0 to 4; p denotes 0 or 1; and R2 denotes a divalent aromatic group. The polymerization degree n is, for example, from 40 to 1000, preferably from 50 to 900, and more preferably from 70 to 800.
When the polyaryletherketone has a repeating structural unit represented by either the formula (1) or (3), the R1 in any of the formulas can be a group represented by a formula (4): 
where F denotes a fluorine atom; z denotes an integer from 0 to 4; and p denotes 0 or 1; R2 denotes a divalent aromatic group.
It is preferable in the formulas (2) and (4) that the divalent aromatic group (R2) is at least one kind of the groups represented by formulas (5): 
where B denotes a halogen atom, a hydrogen atom, a lower alkyl group having 1-4 carbons or a lower alkoxy group having 1-4 carbons.
A specific example of the above-described polyaryletherketone can have a repeating structural unit represented by a formula (6), (7), (8) or (9). The polymerization degree n is, for example, from 40 to 1000, preferably from 50 to 900, and more preferably from 70 to 800. 
Next, a method of producing an optical film according to the present invention includes a step of coating a solution containing polyaryletherketone on a substrate and a step of drying the coated solution. Accordingly, the method can provide an optical film where the refractive index distribution easily can be controlled, and which has high uniformity and a negative birefringence.
Furthermore, a viewing-angle compensating member according to the present invention includes the above-described optical film.
An optical element according to the present invention includes the above-described optical film. The optical element may include at least one additional optical film, and the optical film according to the present invention may be laminated on at least one surface of the additional optical film.
Furthermore, an image display device according to the present invention includes the above-described optical film, the viewing-angle compensating member or the optical element.